The present invention relates to a stencil printing method suitable for obtaining prints on which the ink is smoothly and thickly applied and to an apparatus used for carrying out the method.
Stencil printing is utilized in various fields because of its easy preparation of master stencil sheets. However, according to the stencil printing, a printing ink is applied onto the outer surface of the master stencil sheet superposed on a material to be printed (hereinafter referred to as "printing material") and the printing ink is forced out through perforations of the stencil sheet by an ink supply means such as a pressing plate or a roller to transfer the ink onto the printing material, and thereafter the stencil sheet and the printing material are separated. Therefore, the amount of the printing ink transferred is great, and it is especially difficult to transfer the printing ink at a uniform thickness without causing uneven transfer.
Particularly, uneven transfer of the printing ink is conspicuous in the solid print portion, whereby appearance of printed images is apt to be damaged. This uneven transfer can be improved by using printing materials high in permeability to the printing ink. However, when coated papers, plastics and glass sheets, which are low in permeability to the printing ink and high in smoothness, are used, such improvement cannot be expected. Moreover, if an ink high in fluidity is used, printing high in smoothness is possible thanks to self-leveling of the ink, but it becomes difficult to retain the ink in stencil sheets or to obtain prints onto which the ink is thickly applied.
Causes for such uneven transfer as mentioned above are considered to be as follows. That is, since the ink per se has viscosity, when the stencil sheet and the printing material are separated, an internal stress is generated between the ink on the side of the stencil sheet and the ink transferred to the printing material, and these inks string with each other to cause the ink to finally be cut in pieces irregularly.
For reducing the uneven transfer, Japanese Patent Laid-open (Kokai) No. 61-14978 proposes to make constant a time in which a master stencil sheet is pressed to a printing material to transfer an ink to the material. However, according to this method, unevenness in the amount of the transferred printing ink, which is caused by a difference in the pressing time, can be diminished, but the unevenness on a surface of the transferred printing ink, which is caused by stringiness of the printing ink at the time of separation of the master stencil sheet, cannot be diminished.